Technical Field
The present invention relates to a power electronics unit for driving an electric machine excited by permanent magnets and including a rotor and a stator having at least one stator winding. The power electronics unit comprises at least one power switch designed to drive the stator winding, and an overvoltage protection device for protecting the power switch from overvoltage.
Description of the Related Art
The EMF of machines excited by permanent magnets, i.e., motors and/or generators, has a substantially linear course with the speed of the rotor carrying the magnets. In prior art designs, the maximum EMF is achieved at the maximum speed of the rotor. With the maximum possible phase current of the power electronics system associated with the electric machine, the in theory maximum achievable base power of the machine is thus established by the maximum torque at the upper left point of the power hyperbola and the maximum speed. However, higher speeds can be achieved when the phase angle of the counter voltage impressed by the power electronics system and thus the effective phase current is altered relative to the EMF of the rotor and the machine is operated in a field weakening mode, as it is known especially in electric machines with excitation windings. In case of a machine excited by permanent magnets, the EMF which is effective in the direction of the power electronics system, in case of a field weakening mode of operation, is lowered by the inductive voltage drop in the machine during current flow, and the maximum permissible voltage acting on the power electronics system is reached only at higher rotational speeds. However, this method requires protection mechanisms against sudden failure of the reactive current component, because the inductive voltage drop is not present in this case and the “unmanipulated” EMF is present at the power electronics system. If this occurs in speed ranges where the EMF is higher than the maximum allowable value for the power electronics system, damage to the power electronics system up to partial destruction of the same may result.
EP 0 970 840 and EP 0 974 483 show a protection device against voltage kickback or retroactive voltages in a permanent magnet drive, which is operated in a field weakening mode. The protection device is connected, between an inverter driving the motor and the motor, to the power phases and has a three-phase diode bridge, which has a thyristor connected to its output and a voltage protection electronics system connected in parallel thereto. The protection against excessively high terminal voltages between motor and drive control takes place by igniting the thyristor and the thus resulting short-circuit of all motor terminals via the three-phase diode bridge. The on-resistance of diode bridge and thyristor acts as a braking load in this arrangement.
EP 0 742 637 shows an emergency braking arrangement for realizing an integrated armature short-circuit for an electrical three-phase drive controlled via an inverter for safe braking of the drive in case of a fault that is not described in more detail. For effecting emergency braking in the event of a fault, the control system offers a possibility of bringing about an integrated armature short-circuit by blocking one inverter bridge of the inverter in safe technology, driving the other inverter bridge by clocking driving pulses to cause a short-circuit of the phases of the electric drive. The timing of the clocked driving pulses is selected such that the short-circuit current induced by the armature movement in the armature windings generates an optimum braking moment for the armature.